Intravascular ultrasound (IVUS) imaging is widely used in interventional cardiology as a diagnostic tool for assessing a diseased vessel, such as an artery, within the human body to determine the need for treatment, to guide the intervention, and/or to assess its effectiveness. IVUS imaging uses ultrasound echoes to form a cross-sectional image of a vessel of interest. Typically, an ultrasound transducer on an IVUS catheter both emits ultrasound pulses and receives the reflected ultrasound echoes. The ultrasound waves pass easily through most tissues and blood, but they are partially reflected from discontinuities arising from tissue structures (such as the various layers of the vessel wall), red blood cells, and other features of interest. The IVUS imaging system, which is connected to the IVUS catheter by way of a patient interface module (PIM), processes the received ultrasound echoes to produce a cross-sectional image of the vessel where the transducer is located.
To establish the need for treatment, the IVUS system is used to measure the lumen diameter or cross-sectional area of the vessel. For this purpose, it is important to distinguish blood from vessel wall tissue so that the luminal border can be accurately identified. In an IVUS image, the blood echoes are distinguished from tissue echoes by slight differences in the strengths of the echoes (e.g., vessel wall echoes are generally stronger than blood echoes) and from subtle differences in the texture of the image (i.e., speckle) arising from structural differences between blood and vessel wall tissue. As IVUS imaging has evolved, there has been a steady migration towards higher ultrasound frequencies to improve the resolution. But as ultrasound frequency is increased, there is diminished contrast between the blood echoes and vessel wall tissue echoes. At the 20 MHz center frequency used in early generations of IVUS, the blood echoes were noticeably weak in comparison to the vessel wall echoes due to the small size of the red blood cell compared to the acoustic wavelength. However, at the 40 MHz ultrasound center frequency now commonly used for IVUS imaging, there is only a modest difference between blood and tissue echoes as the ultrasound wavelength approaches the dimensions of a red blood cell.
Another use of IVUS imaging in interventional cardiology is to help identify the most appropriate course of treatment. For example, IVUS imaging may be used to assist in recognizing the presence of a mural thrombus (i.e., coagulated blood attached to the vessel wall and stationary within the blood vessel) in an artery prior to initiating treatment. If a thrombus is identified in a region where disease has caused a localized narrowing of the arterial lumen, then the treatment plan might be modified to include aspiration (i.e., removal) of the thrombus prior to placing a stent in the artery to expand and stabilize the vessel lumen. In addition, the identification of a thrombus could lead the physician to order a more aggressive course of anti-coagulant drug therapy to prevent the subsequent occurrence of potentially deadly thrombosis. In a conventional IVUS image, however, there is very little difference in appearance between a thrombus and moving blood.
Another use of IVUS imaging in interventional cardiology is to visualize the proper deployment of a stent within an artery. A stent is an expandable mesh cylinder that is generally deployed within the artery to enlarge and/or stabilize the lumen of the artery. The expansion of the stent typically stretches the vessel wall and displaces the plaque that otherwise forms a partial obstruction of the vessel lumen. The expanded stent forms a scaffold, propping the vessel lumen open and preventing elastic recoil of the vessel wall after it has been moderately stretched. In this context, it is important to recognize proper stent apposition; that is, the stent struts should be pressed firmly against the vessel wall. A poorly deployed stent may leave stent struts in the stream of the blood flow and these exposed stent struts are prone to initiate thrombus formation. Thrombus formation following stent deployment is referred to as “late stent thrombosis” and these thrombi can occlude the stented location or break free from the stent strut to occlude a downstream branch of a coronary artery and trigger a heart attack.
In these examples of IVUS imaging, it is particularly useful to identify moving blood, and to distinguish the moving blood from the relatively stationary tissue or thrombi. Motion information can be helpful in delineating the interface between blood and vessel wall so that the lumen border can be more easily and accurately identified. Motion parameters such as velocity may be the most robust ultrasound-detectable parameters for distinguishing moving blood from a stationary thrombus. In the case of stent malapposition, the observation of moving blood behind a stent strut is a clear indication that the stent strut is not firmly pressed against the vessel wall as it should be, possibly indicating a need to further expand the stent. In each of the aforementioned IVUS imaging examples, the addition of motion parameters to the traditional IVUS display of echo amplitude can improve the diagnosis and treatment of a patient.
There are two types of IVUS catheters in common use today: solid-state and rotational, with each having advantages and disadvantages. Solid-state IVUS catheters use an array of ultrasound transducers (typically 64) distributed around the circumference of the catheter and connected to an electronic multiplexer circuit. The multiplexer circuit selects array elements for transmitting an ultrasound pulse and receiving the echo signal. By stepping through a sequence of transmit-receive pairs, the solid-state IVUS system can synthesize the effect of a mechanically scanned transducer element, but without moving parts. Since there is no rotating mechanical element, the transducer array can be placed in direct contact with the blood and vessel tissue with minimal risk of vessel trauma and the solid-state scanner can be wired directly to the imaging system with a simple electrical cable and a standard detachable electrical connector.
In a typical rotational IVUS catheter, a single ultrasound transducer element fabricated from a piezoelectric ceramic material is located at the tip of a flexible driveshaft that spins inside a plastic sheath inserted into the vessel of interest. The transducer element is oriented such that the ultrasound beam propagates generally perpendicular to the axis of the catheter. The fluid-filled sheath protects the vessel tissue from the spinning transducer and driveshaft while permitting ultrasound waves to freely propagate from the transducer into the tissue and back. As the driveshaft rotates (typically at 30 revolutions per second), the transducer is periodically excited with a high voltage pulse to emit a short burst of ultrasound. The same transducer then listens for the returning echoes reflected from various tissue structures, and the IVUS imaging system assembles a two dimensional display of the vessel cross-section from a sequence of several hundred of these ultrasound pulse/echo acquisition sequences occurring during a single revolution of the transducer.
While the solid-state IVUS catheter is simple to use, thanks to its lack of moving parts, it cannot match the image quality available from a rotational IVUS catheter. It is difficult to operate a solid-state IVUS catheter at the same high frequency as a rotational IVUS device, and the lower operating frequency of solid-state IVUS catheters translates into poorer resolution compared to that of a higher frequency rotational IVUS catheter. There are also artifacts such as sidelobes, grating lobes, and poor elevation focus (perpendicular to the imaging plane) that arise from the array-based imaging that are greatly reduced or completely absent with a rotational IVUS device. Despite the image quality advantages of the rotational IVUS catheter, each of these devices has found a niche in the interventional cardiology market, with solid-state IVUS preferred in circumstances where ease-of-use is paramount and the reduced image quality is acceptable for the particular diagnostic needs, while rotational IVUS is preferred where image quality is paramount and the more time-consuming catheter preparation is justified.
Traditionally, IVUS catheters, whether rotational or solid-state catheters, are side-looking devices, wherein the ultrasound pulses are transmitted substantially perpendicular to the axis of the catheter to produce a cross-sectional image representing a slice through the blood vessel. The blood flow in the vessel is normally parallel to the axis of the catheter and perpendicular to the plane of the image. IVUS images are typically presented in a grey-scale format, with strong reflectors (vessel boundary, calcified tissue, metal stents, etc.) displayed as bright (white) pixels, with weaker echoes (blood and soft tissue) displayed as dark (grey or black) pixels. Thus, flowing blood and static blood (i.e., thrombi) may appear very similar in a traditional IVUS display.
In ultrasound imaging applications, Doppler ultrasound methods are often used to measure blood and tissue velocity, and the velocity information is used to distinguish moving blood echoes from stationary tissue echoes. Commonly, the velocity information is used to colorize the grey-scale ultrasound image in a format referred to as Doppler color flow ultrasound imaging, with fast moving blood tinted red or blue, depending on its direction of flow, and with slow moving or stationary tissue displayed in grey-scale.
Traditionally, IVUS imaging has not been amenable to Doppler color flow imaging since the direction of blood flow is predominantly perpendicular to the IVUS imaging plane. More specifically, Doppler color flow imaging and other Doppler techniques do not function well when the velocity of interest (i.e., blood flow velocity) is perpendicular to the imaging plane and perpendicular to the direction of ultrasound propagation, resulting in near zero Doppler shift attributable to blood flow. In the case of rotational IVUS, there is an added complication due to the continuous rotation of the transducer, which makes it problematic to collect the multiple echo signals from the same volume of tissue needed to make an accurate estimate of the velocity-induced Doppler shift.
In the case of solid-state IVUS, the problem of low Doppler shift has been overcome to some extent by the development of an alternative (non-Doppler) method for blood motion detection. The ChromaFlo method (U.S. Pat. No. 5,921,931) uses an image correlation method instead of Doppler to identify moving blood. Image correlation techniques for motion detection are generally inferior to Doppler methods, and in particular, are not suitable for rotational IVUS since the rate of decorrelation attributable to the rotating ultrasound beam is comparable to the rate of decorrelation due to the blood flow. Solid-state IVUS catheters avoid this rotating beam problem by electronically maintaining a constant beam direction for a sequence of pulses before electronically incrementing the beam direction to the next image angle.
In U.S. Provisional Patent Application No. 61/646,080 entitled “Device and System for Imaging and Blood Flow Velocity Measurement,” filed May 11, 2012 and incorporated by reference herein in its entirety, there is described a rotational IVUS catheter configuration and an IVUS imaging system architecture capable of overcoming the aforementioned obstacles to Doppler color flow imaging. A key aspect of the invention is that the ultrasound transducer is tilted such that the ultrasound beam emerges from the catheter at a substantial angle with respect to a perpendicular to the catheter axis.
In U.S. Provisional Patent Application No. 61/646,062 entitled “Circuit Architecture and Electrical Interface for an Advanced Rotational IVUS Catheter” filed on May 11, 2012 and incorporated by reference herein in its entirety, there is further described an advanced transducer technology capable of providing superior IVUS image quality compared to that available from the traditional rotational IVUS catheter utilizing lead-zirconate-titanate (PZT) piezoelectric ceramic transducer technology. The piezoelectric micromachined ultrasound transducer (PMUT) fabricated using a polymer piezoelectric material, also disclosed in U.S. Pat. No. 6,641,540, hereby incorporated by reference in its entirety, offers greater than 100% bandwidth for optimum resolution in the radial direction, and a spherically-focused aperture for optimum azimuthal and elevation resolution. While this polymer PMUT technology promises significant image quality advantages, the inherently planar silicon wafer fabrication process for manufacturing these advanced transducers makes it difficult to achieve the substantial tilt angle required for Doppler color flow imaging in the relatively small area available with an IVUS catheter.
Accordingly, there is a need for improved devices, systems, and methods for providing a polymer piezoelectric micro-machined ultrasonic transducer and rotational IVUS catheter configuration providing the required transducer tilt angle and other features to render it suitable for use with a Doppler color flow intravascular ultrasound imaging system.